Semi-solid metal forming, i.e., forming a metallic alloy at a temperature between its equilibrium liquidus and equilibrium solidus temperatures, is a hybrid metalworking process combining the elements of both casting and forging/extrusion. One of the key elements for the successful operation of a semi-solid forming process is the microstructure of metallic alloy being thus formed.
Conventionally solidified metals cannot be utilized in a semi-solid condition, since a structure of dendritic network forms upon solidification in such metals. Cracks and segregates will occur when a conventionally solidified metal is formed in partially liquid/solid state. Previous studies have shown that raw material of a semi-solid forming process must have a structure comprised of globular or spheroidal grains contained in a lower melting alloy matrix. When heated to a semi-solid temperature, the globular solid phase is retained, suspended in a lower melting alloy liquid matrix. The term "metal" is used to designate a metallic alloy with a major metallic constituent (base metal) along with various amounts of intentional additions (metallic and non-metallic) that modify the property of the base metal, as well as trace impurities that are deemed to not greatly deteriorate the performance of the alloy when used to fabricate articles thereof.
Producing semi-solid raw material requires specialized techniques. Thermal transformation processes are disclosed in U.S. Pat. Nos. 4,106,956, 5,009,844 and 5,571,346 where solidified metal having a fine dendritic microstructure is heated to and maintained at a superheated temperature above the solidus temperature of the metal, while keeping its body in a solid shape. After the dendritic networks are thermally transformed into globular solid particles, the metal is then formed in semi-solid conditions into an article.
Vigorous agitation processes are disclosed in U.S. Pat. Nos. 3,902,544, 3,948,650, 3,954,455, 4,310,352 (mechanical stirring) and U.S. Pat. No. 4,229,210 (inductive electromagnetic stirring) where during billet casting, a metal is agitated while it is in the semi-solid state and then cooled to solidify, forming the primary solid phase comprising discrete degenerate dendrites or nodules while preventing the formation of interconnected dendritic networks. Among the various agitation processes, the magnetohydro-dynamic (MHD) casting process has been commercially applied for producing a variety of fine-grain (mean grain effective diameter about 30 .mu.m) aluminum alloy bars (diameters varying from 38 to 152 mm) which satisfy the requirements of semi-solid forming.
However the agitation processes have practical limitations for casting bars with diameters less than about one inch due to very low productivity. U.S. Pat. No. 4,415,374 discloses a "SIMA" (strain induced, melt activated) process to make raw material for semi-solid forging. In the process, a solid metal composition is prepared by heating a conventionally solidified and homogenized ingot to a temperature in the hot deformation range of the metal, followed by hot extrusion or hot rolling plus additional cold working, resulting in an essentially directional grain structure. By heating the composition to a temperature above the solidus and below the liquidus, its directional grain structure transforms to a partially solid, partially liquid mixture comprising of uniform discrete spheroidal particles contained in a lower melting liquid matrix. The heated alloy is then formed and solidified while in a partially solid, partially liquid condition, the solidified article having a uniform, fine grained microstructure.
In comparison with MHD casting, the SIMA process provides an effective method for producing small-diameter alloy bars (diameter less than 38 mm or 1.5 in.) employed in semi-solid forging. For large sizes, however, the economics of the process are not competitive with those of MHD casting for most metal alloys. On the other hand, the procedure of the SIMA process is very cumbersome, comprising the five discrete operations: conventional casting, ingot homogenization, heating, hot working and cold working. The nature of the process limits its application on a practical and economical scale, for not only large size but also small size semi-solid raw materials. In addition, the requirement of cooling to room temperature and reheating for further processing is very costly.
Therefore, it is an object of the present invention to provide a more superior process for producing a fine-grained solid metal composition suitable for semi-solid metal forming.
It is another object of the present invention to provide a more economical process for producing a fine-grained solid metal composition suitable for semi-solid metal forming.
It is another object of the present invention to provide a process and apparatus for preparing semi-solid precursor material covering a large range of sizes.
It is a further object of the present invention to provide a process and apparatus for preparing, delivering and semi-solid forming the above precursor material.